Aliens


 * There are two kinds of alien: regular and Alien Queen, who can lay eggs. Eggs will have after a few turns into regular aliens.
 * Depending on the mission, if the queen is killed, a random regular alien is chosen and turns into the queen.
 * Depending on the mission, if an alien kills an enemy human unit, then after a few turns, another alien will be hatched.
 * Aliens are affected by nerve gas.